This invention relates to a circuit arrangement to protect a remotely controllable semi-conductor switch, which serves to close and interrupt an alternating current circuit, against overload.
With semi-conductor switches of the above type, there is a danger of destruction if the load lying in the alternating current circuit is by-passed or bridged while the switch is closed, such as is the case in a short-circuit. The current through the switch is then only limited by the inner resistance of the voltage source and the dynamic resistance of the switch. This limit value, however, generally lies far in excess of the maximum permissible current for the semi-conductor switch.
In order to limit the current enough, in the case of a short circuit, so that it can still be disconnected from the semi-conductor switch, it is known to insert a protective resistor in the electrical circuit. A protective resistor of this type, however, does not increase only the circuit expense. Above all, it results in a relatively large amount of waste heat, which is disadvantageous for several reasons. More favorable, therefore, is a different known solution (DE-OS No. 25 45 919), in which as current limiting device with variable resistance and a current monitoring device lie in the path of the current of the semi-conductor switch, which is formed as a thyristor. As long as the nominal current is not exceeded, the resistance of the current limiting device is relatively slight. It does not increase automatically in case of a short circuit until a value is reached which limits the short-circuit current by means of the thyristor to a value at which switching can still take place. The current monitoring device acts in case of a short circuit on the trigger of the thyristor and closes the starting voltage. The expense of the circuit for this solution, however, is relatively large.
The circuit expense is smaller in another known circuit arrangement of the above-described, type (DE-OS No. 31 15 214). Here a MOS-FE is connected in series with the thyristor forming the semi-conductor switch and a solid resistor necessary for the current monitoring. This MOS-FET is switched into the blocked condition in case of a short circuit or when there is a time delay at excess current, thereby practically blocking the path of the current in which the thyristor lies. The disadvantage of this solution is that a thyristor is necessary to control the MOS-FET which has to be triggered if the MOS-FET is to be switched into the blocked condition. The triggering voltage of thyristors is relatively large. To the extent that protection is to be provided not only for the case of a short circuit, but also for cases of excess current, a relatively larger margin of safety from the maximum permissible excess current must be maintained. In addition, a thyristor cannot be integrated into a miniaturized circuit because of the space it requires.